A widely-used method for molding plastic is to extrude high-temperature molten plastic in a tubular shape, enclose this in a metal mold and make the tube expand by blowing air into it. A prior art example of this will be explained with reference to FIG. 10, which shows the process of manufacturing a container by blow molding. As shown in FIG. 10(a), molten plastic in tubular shape (the parison) is extruded into the middle of a split metal mold, and as shown in FIG. 10(b), the metal mold is then closed. As shown in FIG. 10(c), when air is blown into the molten plastic, the plastic adheres closely to the inner wall of the metal mold and assumes the same shape as this inner wall. At this point in time the molten plastic is at a high temperature of for example 200.degree. C., and it is cooled and solidified by keeping it in the metal mold while continuing to blow in high-pressure air. Although the time required for this cooling varies according to the type of plastic and the form of the molding, cooling time has hitherto been determined by aiming for a temperature at which the deformation due to thermal shrink of the resin after taking out from the metal mold is linear. As shown in FIG. 10(d), when the metal mold is opened, the molding is took out.
The molding shown in FIG. 10 is a container (a bottle) which will be marketed after being filled with a liquid. The usual aim is to open the metal mold when the resin has cooled to about 50.degree. C. A dozen or so seconds are required for this cooling.
As previously mentioned, it takes time to cool the high-temperature molten plastic to a point at which the metal mold can be opened. Production per unit time and production cost are in proportion, and in a manufacturing process in which time management in carried out in units of seconds, even a short cooling time of a dozen or so seconds should be shortened in order to achieve lower production cost.
In order to obtain data on the shortest practical cooling time, the inventors therefore performed repeated experiments in which a metal mold was opened while still at a high temperature. As a result, it was found that if a metal mold is separated before the conventionally employed cooling time has elapsed, the high-temperature molten plastic contracts greatly and undergoes nonlinear deformation. Accordingly, the targeted molding shape could not be obtained and cooling time could not be shortened.
A container manufactured by above-mentioned blow molding is pasted a label after cooling. While, it is also used a method that a label may be pasted in the molding process. This method is generally called as "in-mold labelling".
In the in-mold labelling, a label made by synthetic resin is placed on the inside of the metal mold before molding process and is pasted with the container by the heat of the molten plastic on the molding process so that the container and the label are formed as one body.
In the in-molding labelling, it is formed different two kinds of materials overlapping so that it is more difficult to predict deformations caused by the different heat shrinkage rates of the materials than to do for a container with no in-mold label.
In a prior method for manufacturing a container with an in-mold label, it is proposed a method for compensating deformations which is based on the view that the label face is so thin and becomes cool so rapidly because of it being contacted with the metal mold, that it happens shrinkage on more the container side than the label face after having taken it out from the metal mold and that the label face deforms (swells out) to the outside of the container
However, deformation of a label face does not only depend on difference of shrinkage rates of the label material and the container material. For example, a label face bears not only the shrinkage of a container but also compression load from more than one directions parallel to the label face. Consequently, a label face does not always deform (swell out) to outside of the container.
In other words, a label face swells out (deforms to outside) by the bending effect caused by the difference of temperature of the resin thickness direction. However, the label face collapses (deforms to inside) by the shrinkage difference in the surrounding direction caused by the thickness distribution in the bottle height direction. The label face may collapse or swell out in depending on the initial condition of the container by the shrinkage difference of the label material and the container material.
The present invention has been devised in the light of this background. It is an object of this invention to provide a method and apparatus for designing a metal mold whereby a targeted molding shape can be obtained even though the cooling time in the plastic molding process is shortened. It is a further object of this invention to provide a method and apparatus for designing a metal mold whereby a molding can be manufactured with a shape which compensates for the deformation due to hydrostatic pressure resulting from the molding being filled with contents. It is yet another object of this invention to provide a method and apparatus for designing a metal mold whereby the metal mold can be designed without dependence on prior experience.